According to a conventional expansion molding method and apparatus, the manufacture of molded articles of expanded resin includes the following steps: filling a cavity in a mold with expandable thermoplastic resin beads (hereinafter referred to as expandable resin beads) which have been expanded in advance to have a specified bead diameter; heating the expandable resin beads to be fused together; and cooling the resulting expandable resin beads to produce a molded article of expanded resin. Here, the cavity which becomes the external frame of the molded article is required to have a width (thickness) greater than the diameter of the pre-expanded expandable resin beads so that the cavity is filled up with the expandable resin beads.
Some molded articles are required to have thin walls for their uses. With the above-mentioned method and apparatus, however, when a cavity whose width is smaller than the diameter of a pre-expanded expandable resin bead, for example about 5 mm, is required, the cavity can not be filled with the expandable resin beads.
To overcome such a drawback, for example, Japanese Publication for Examined Patent Application No. 53-30741/1978 discloses an expansion molding method. In this method, as shown in FIG. 21 and FIG. 22, a cavity 53 formed between a male mold 51 and a female mold 52 is filled with expandable resin beads while partially expanding its capacity with adjusting members 54. Then, the expandable resin beads are compressed by the adjusting members 54 to reduce the capacity of the cavity 53. With this arrangement, it is possible to manufacture thin-walled molded articles of expanded resin by compressing the resin beads with the adjusting members 54 until the width of the cavity 53 becomes smaller.
This expansion molding method is effective when manufacturing molded articles of expanded resin having partially thin walls. However, the installation of the adjusting members 54 is difficult in the arrangement for producing molded articles which have, for example, consistently thin walls, and also causes a mold with a complicated structure. Moreover, when manufacturing molded articles which have, for instance, a cylindrical shape like cups, it is impossible to reduce the thickness of the side walls consistently with the adjusting members 54.
To solve such a problem, for example, Japanese Publication for Examined Patent Application No. 62-10175/1987 and Japanese Publication for Unexamined Patent Application No. 63-153119/1988 disclose expansion molding methods and apparatuses for producing molded articles of expanded resin which have consistently thin side walls.
More specifically, with the expansion molding method of Application No. 62-10175/1987, as illustrated in FIG. 23, cracking is arranged, i.e. the mold is closed while leaving a gap 56a between a male mold 55 and a female mold 56 when filling the cavity 57 with the expandable resin beads. Here, the width of the gap 56a is smaller than the diameter of an expandable resin bead, but the width of a cavity 57 formed between the male mold 55 and the female mold 56 is greater than the diameter of the expandable resin bead. After filling the cavity 57 with the expandable resin beads, the mold is completely closed. This process enables the cavity 57 to be sufficiently filled up with the expandable resin beads, and thereby reducing the thickness of the side wall.
Meanwhile, as illustrated in FIG. 24, the method and apparatus of Application No. 63-153119/1988 has a finish-forming mold for forming finished articles and a preforming mold for forming preformed articles which have a greater thickness compared to the finished articles. The finish-forming mold is composed of a male mold 61 and a female mold 59, while the preforming mold is composed of a male mold 60 and a female mold 58. When manufacturing molded articles, firstly preformed articles are formed by filling up the cavity in the preforming mold with expandable resin beads, and the preformed articles are then compressed in the finish-forming mold to reduce the wall thickness. This method also makes it possible to produce finished articles having consistently thin walls like the above-mentioned method of Application No. 62-10175/1987.
However, with the methods and apparatuses disclosed in the above-mentioned applications No. 62-10175/1987 and No. 63-153119/1988, after filling the cavity with expandable resin beads or producing the preformed article, the thickness of the molded article is reduced by moving the mold in the same direction as the closing direction. Therefore, the portions of the cavity, which become the walls of the molded article, to be thinned need to slope with respect to the closing direction.
For example, as shown in FIG. 25, when the side faces of circumference sections 64a of a cavity 64 formed between a male mold 62 and a female mold 63 extend parallel to a closing direction P, the widths of the circumference sections 64a will not change even when the above-mentioned cracking is performed, and therefore the circumference sections 64a can not be filled up with expandable resin beads 65. Besides, the process for forming a preformed article into a finished article in the finish-forming mold is infeasible as the external diameter of the preformed article is greater than the internal diameter of the opening of the finish-forming mold.
As describe above, with the conventional expansion molding methods and apparatuses, the thickness of the side walls is limited according to the diameter of expandable resin beads, preventing the use of expandable resin beads of great diameters. In addition, when forming molded articles having consistently thin side walls, requirements are, for example, the side walls need to slope imposed, thereby causing limited variations in shapes of the molded articles.